|Mountain Time Zone|
|8:47 pm, 21 February 2019 MST|
|Observance of DST|
|DST is observed in some of this time zone.|
The Mountain Time Zone of North America keeps time by subtracting seven hours from Coordinated Universal Time (UTC) when standard time is in effect, and by subtracting six hours during daylight saving time (UTC−06:00). The clock time in this zone is based on the mean solar time at the 105th meridian west of the Greenwich Observatory. In the United States, the exact specification for the location of time zones and the dividing lines between zones is set forth in the Code of Federal Regulations at 49 CFR 71.[a]
In the United States and Canada, this time zone is generically called Mountain Time (MT). Specifically, it is Mountain Standard Time (MST) when observing standard time, and Mountain Daylight Time (MDT) when observing daylight saving time. The term refers to how the Rocky Mountains, which range from northwestern Canada to the US state of New Mexico, are located almost entirely in the time zone. In Mexico, this time zone is known as the Zona Pacífico (Pacific Zone). In the US and Canada, the Mountain Time Zone is to the east of the Pacific Time Zone and to the west of the Central Time Zone.
In some areas, starting in 2007, the local time changes from MST to MDT at 2 am MST to 3 am MDT on the second Sunday in March and returns at 2 am MDT to 1 am MST on the first Sunday in November.
Sonora in Mexico and most of Arizona in the United States do not observe daylight saving time, and during the spring, summer, and autumn months they are on the same time as Pacific Daylight Time. The Navajo Nation, most of which lies within Arizona, does observe DST, although the Hopi Nation, as well as some Arizona state offices lying within the Navajo Nation, do not.
The largest city in the Mountain Time Zone is Phoenix, Arizona. The Phoenix metropolitan area is the largest metropolitan area in the zone; the next largest metropolitan area that observes Mountain Time is Denver, closely followed by the El Paso–Juárez area. TV broadcasting in the Mountain Time Zone is typically tape-delayed one hour, so that shows match the broadcast times of the Central Time Zone (i.e. prime time begins at 7 pm MT following the same order of programming as the Central Time Zone).
Only one Canadian province is fully contained in the Mountain Time Zone:
One province and one territory are split between the Mountain Time Zone and the Pacific Time Zone:
One territory and one province are split between the Mountain Time Zone and the Central Time Zone
The following states have the same time as Mountain Time Zone:
- Baja California Sur
- Nayarit: Except for the Bahía de Banderas municipality, which uses the Central Time Zone.
- Sonora – no daylight saving time, always on MST.
- Revillagigedo Islands (Colima): three of the four islands have the same time as Mountain Time Zone, Isla Socorro, San Benedicto Island and Roca Partida.
Five states are fully contained in the Mountain Time Zone:
Four states are split between the Mountain Time Zone and the Pacific Time Zone. The following locations observe Mountain Time:
- Arizona – though fully in the Mountain Time Zone, clocks are essentially on the same time as Pacific Daylight Time in the summer. The Navajo Nation, however, does observe Daylight Saving Time, and its clocks are aligned with Mountain Daylight Time in the summer.
- Idaho – Southern Idaho
- Nevada – West Wendover is officially on Mountain Time; Mountain City, Owyhee, Jackpot, and Jarbidge observe Mountain Time unofficially
- Oregon – the majority of Malheur County
Six states are split between the Mountain Time Zone and the Central Time Zone. The following locations observe Mountain Time:
- Kansas – Sherman, Wallace, Greeley and Hamilton counties
- Nebraska – western third
- North Dakota – the SW corner counties (Adams, Billings, Bowman, Golden Valley, Grant, Hettinger, Slope, Stark) observe MST. The counties of McKenzie, Dunn, and Sioux counties are split.
- Oklahoma – Kenton (de facto)
- South Dakota – western half
- Texas – El Paso and Hudspeth and northwestern Culberson County, Texas
- "49 CFR 71.8 Mountain zone". Code of Federal Regulations. Retrieved October 7, 2011.
- "49 CFR 71.7 Boundary line between central and mountain zones". Code of Federal Regulations. Retrieved October 7, 2011.
- "49 CFR 71.9 Boundary line between mountain and Pacific zones". Code of Federal Regulations. Retrieved October 7, 2011.
- Robbins, Ted (March 11, 2007). "Arizona Says No to Daylight-Saving Time". National Public Radio. Retrieved June 18, 2012.